Dcsvrljitinii, uj (renera find Sj^ecies. ^M 



applanatiuii the remains are seldom fossilised sideways, and, owinii to the verj- prii-klj- 

 back and the prominent ridue keel which cause it to take a strong hold of the 

 embedding sediments, the fossil (^ften splits so as to show the ventral side. When the 

 splitting takes place along the back, the tips of the spines and the crests of the keels 

 remain embedded in the counterpart, and thus the test seems to be perforated by 

 iiniumerable more or less nearly circular holes, and the serration of the crests is not 

 apparent. The points of the spines on the carapace are all directed forward. This is most 

 apparent in the serrations of the lateral keels, which are really oidy flattened spines. 

 The spines found on the posterior segments of the tail and the tail fan are all directed 

 backwards, showing that the animal kept its powerful tail in reserve for darting 

 suddenly backwards, as in the recent Macrura. The carapace (fig. 1) T. 2763", as seen 

 flattened, is subquadrate, a little longer than broad, with somewhat bulging sides. The 

 anterior margin is slightly coi^vex and is produced into a long pointed and serrated 

 rostrum, while the posterior margin is slightly concave. The antero-lateral angles are 

 each marked by a strong, forwardly-directed, tiattened spine. The apparent lateral 

 margins are serrated by a series of smaller spines back to the postero-lateral angles, the 

 posterior margin here making nearly a right angle with the sides and being edged by a 

 double smooth band, with a deep median groove somewhat like an ogee moulding. 

 There is good reason to consider that what appears as the lateral margin in the fossils 

 is not the true edge of the carapace. Ijut merely a serrated, strongly-developed, short 

 longitudinal keel, and that the true edge bordered by the usual plain band, folded 

 inwards during the life of the animal, became crushed under the carapace on being 

 embedded. An indication of this is shown in the specimen from which fig. 1 was 

 taken, where part of the test is seen to pass inwards for a short distance from the 

 posterior angles, and this is of common occurrence in the fossils. The infold of the 

 carapace is even better shown in fig. 2, T. 27(;(3", which is a view from beneath of a 

 specimen preserved back down. On the left side of the animal (the right haiid of the 

 beholder) a pellicle of sediment is seen to be interposed between the broken limbs and the 

 underside of the carapace. That it is the underside is shown by the spines which stud 

 its back, showing as retreating fuiuiel-shaped hollows ; but there is a belt of test just 

 within the serrated margin studded with smaller spines which stand out towards the 

 observer showing that it is the outside aspect of the carapace. It is therefore highly 

 probable that the doublure may account for the earlier figures of this form and of 

 kindred species, showing the serrations along the anterior part of the lateral margins 

 only. Fig. 4, T. 2753'", takeii from a specimen in the Geological Survey collection, illus- 

 trates this point and shows how this could easily happen. On the left side, the carapace 



